2. CATALOGUES OF GALAXIES, CLUSTERS AND SUPERCLUSTERS

Our understanding of the structure of the Universe is based on the
distribution of galaxies. Until the mid-1970s the number of galaxies
with known distances (redshifts) was very small, thus conclusions on
the structure were based on counts of galaxies. The largest of such
counts was compiled in Lick Observatory by Shane & Virtanen
[51].
This catalogue was analyzed by Seldner
[49]
and played a crucial role in the development of the
hierarchical clustering scenario of structure formation by Peebles
[42].

A big step in the study of the clustering of galaxies and clusters of
galaxies was made by visual inspection of the Palomar Observatory Sky
Survey plates with the aim to produce catalogues of galaxies and
clusters of galaxies. The first of these catalogues was prepared by
Abell [1]
for clusters of galaxies. This catalogue covers the
sky north of declination
-27°. Abell, Corwin & Olowin
[2]
extended the cluster catalogue to the southern sky. Both
these catalogues together contain 4074 clusters. A much larger
catalogue was compiled by Zwicky
[63]; in this catalogue
all galaxies brighter than photographic magnitude
mph 15.7
as well as clusters of galaxies north of declination
-2.5°
are listed. Abell and Zwicky used rather different definitions of
clusters. Abell clusters contain at least 30 galaxies in a magnitude
interval of
m = 2, starting from the
third brightest galaxy,
and located within a radius of 1.5 h-1 Mpc (we use in
this paper the
Hubble constant in units
H0 = 100 h km s-1 Mpc-1).
Distances of clusters were estimated on the basis of the brightness of
the 10th brightest galaxy. Clusters were divided to richness and
distance classes. Zwicky used a more relaxed cluster definition, with
at least 50 galaxies in a magnitude interval of
m = 3,
starting from the brightest galaxy, located within a contour where the
surface density of galaxies exceeded a certain threshold. Due to
these differences some Zwicky clusters are actually central parts of
superclusters which contain several Abell clusters and groups of
galaxies (an example is the Perseus cluster). Since the definition of
clusters in the Abell catalogue is more exact, this catalogue has
served for a large number of studies of the structure of the Universe.
On the other hand, the Zwicky catalogue of galaxies was the basic
source of targets for redshift determinations.

An early catalogue of bright galaxies was compiled by Shapley & Ames
[53].
Sandage & Tammann
[48]
published a revised
version of this catalogue; it contains data on galaxies brighter than
13.5 magnitude, including redshifts. This catalogue, and the
compilation of all available data on bright galaxies by de
Vaucouleurs, de Vaucouleurs, & Corwin
[18]
were the sources
of distances which allowed to obtain the first 3-dimensional distributions
of galaxies. Much more detailed information on the spatial
distribution of galaxies was obtained on the basis of redshifts,
measured at the Harvard Center for Astrophysics (CfA) for all Zwicky
galaxies brighter than
mph = 14.5. Later this survey was extended
to galaxies brighter than
mph = 15.5 (the second CfA catalogue),
and to galaxies of the southern sky (Southern Sky Redshift
Survey) [14].

These early redshift compilations made it possible to discover the
filamentary distribution of galaxies and clusters forming huge
superclusters, as well as the absence of galaxies between them. These
results were first reported in the IAU Symposium on Large-Scale
Structure of the Universe
[31,
55,
57,
58] and
demonstrated that the pancake scenario of structure formation by
Zeldovich [60,
61]
fits observations better than the
hierarchical clustering scenario. More detailed studies of the
structure formation by numerical simulations showed that the original
pancake scenario by Zeldovich also has weak points - there is no fine
structure in large voids between superclusters observed in the real
Universe [62]
and the structure forms too late
[16],
thus a new scenario of structure formation was suggested based on the
dominating role of the cold dark matter in structure evolution
[5].
In a sense the new scenario is a hybrid between the
original Peebles and Zeldovich scenarios: structure forms by
hierarchical clustering of small structures within large filamentary
structures - superclusters.

The next big step in the study of the large-scale distribution of
galaxies was made on the basis of the catalogue of galaxies formed on
the basis of digitized images of the ESO Sky Survey plates using the
Automated Plate Measuring (APM) Facility
[36,
37]. The APM
galaxy catalogue covers 185 ESO fields, is complete up to magnitude
bj = 20.5, and was the basis for a catalogue of
clusters prepared
by Dalton [15].
The analysis of the APM galaxy sample
showed that properties of the distribution of galaxies differ from the
standard CDM model which assumed that the density of matter is equal
to the critical density. A low-density model with cosmological term
(dark energy) fits the data better
[19].

The modern era of galaxy redshift catalogues started with the Las
Campanas Redshift Survey (LCRS). Here, for the first time,
multi-object spectrographs were used to measure simultaneously
redshifts of 50 - 120 galaxies
[52].
The LCRS covers 6
slices of size
1.5 × 80 degrees, the total number of galaxies
with redshifts is
~ 26, 000, and the limiting magnitude is
bj = 18.8. Presently several very large programs are
under way to
investigate the distribution of galaxies in a much larger volume. The
largest project is the Sloan Digital Sky Survey (SDSS), a cooperative
effort of several North-American institutions with participants from
Japan [34].
This survey covers the whole northern sky and a
strip in the southern sky. The sky is first imaged in five photometric
bands to a limiting magnitude about 23 (the limit varies with spectral
bands), thereafter redshifts are measured for all galaxies up to a
magnitude ~ 18, and active galactic nuclei (AGN) up to ~ 19;
additionally a volume-limited sample of redshifts of bright
elliptical galaxies is formed. The total number of galaxies with
measured redshifts will probably exceed one million. Another large
redshift survey uses the 2-degree-Field
[35] spectrograph of
the Anglo-Australian Telescope. This survey is based on the APM
galaxy catalogue and covers two large areas of size
75° × 12.5° and
65° × 7.5° with limiting
magnitude
bj
19.5. The goal is to measure about 250,000
redshifts. It is expected that new redshift surveys give us the
possibility to investigate the detailed structure of the Universe up
to a distance of
2000 h-1 Mpc.

The largest systems of galaxies are superclusters, which are defined
as the largest systems of galaxies and clusters still isolated from
each other. Catalogues of superclusters have been constructed using
Abell clusters of galaxies. The latest compilation by Einasto
[26] contains 220
superclusters with at least two member clusters.